Abstract: Ru-based oxygen evolution reaction (OER) catalysts exhibit considerable promise for the replacement of Ir-based catalysts due to their high activity and relatively low cost. However, optimization of activity and stability of Ru-based OER catalysts in acidic environment is still a challenging task. Here, we present an optimized amorphous-rutile crystalline heterostructure Ru₃Mn₁O_x-250 catalyst could achieve OER activity with an overpotential of only 211 mV at 10 mA/cm² while maintaining stable operation for at least 1000 h in acidic electrolyte. When the catalyst was used as an anode material in a proton exchange membrane (PEM) electrolyzer, it could deliver an industrial current of 1 A/cm² at 1.65 V (80 °C), outperforming the commercial RuO₂ catalyst (1.82 V). The catalyst can even maintain stable operation at 1 A/cm² for 100 h, showcasing its high OER activity and stability. The experimental and theoretical studies revealed that Mn is atomically dispersed throughout the amorphous-crystalline phases mainly in form of low valence state Mn, forming asymmetric Ru-O-Mn bonds which leads to distorted Oh coordination geometry of Mn with Ru. Such unique microstructure leads to: (1) enhancement of OER activity by reduction of the d band center further away from the Fermi level, weakening the adsorption of oxygen intermediates and accelerating the rate-determining *OOH intermediate formation; (2) enhancement of the catalyst stability by increasing the energy barrier of *RuO(OH)₂ formation, which is the key intermediate for the catalyst dissolution via RuO₄^- formation. This work demonstrates that an amorphous-crystalline heterostructure design strategy is an effective way to overcome the activity-stability trade-off, offering a new approach for the development of efficient OER catalysts stable in acidic electrolyte.

Key words: Ruthenium-manganese oxide, Amorphous-crystalline structure, Oxygen evolution reaction, Acidic electrolyte, Proton exchange membrane water, electrolysis